The present invention relates to a method to generate a neutral molecular beam of sample molecules by multistage spraying of the sample solution and further to an apparatus that actualizes this method.
The conventional method for generating a molecular beam comprises steps of: mixing gaseous sample molecules with rare gas atoms; introducing the mixed gas through a nozzle directly into vacuum where the mixed gas is adiabatically expanded to form a supersonic jet flow; and guiding this supersonic jet flow through a skimmer to form a molecular beam. For a sample in form of liquid or solid, the method similar to what described above for the gaseous sample is carried out after heating the sample so as to vaporize or to sublime.
The sample molecules in the supersonic jet flow are adiabatically expanded in vacuum, so that they are cooled at a temperature of several Kelvins for rotation and at a temperature of several dozens of Kelvins for vibration. Consequently, the sample molecules occupy the ground states and therefore a rotational energy distribution of the sample molecules is simplified.
FIG. 3 illustrates an example of the conventional method. A mixed gas (32) composed of gaseous sample molecules and rare gas atoms is ejected through an orifice (34) from a mixed gas reservoir (33) into a vacuum chamber in which the mixed gas is to be adiabatically expanded. The mixed gas in form of a supersonic jet flow is then guided through a skimmer (35) to generate the molecular beam (36). Shock waves, such as Barrel shock wave and Mach disc shock wave, are generated as the mixed gas (32) is ejected through the orifice (34) or the nozzle.
For a liquid or solid sample, the method similar to what is previously described is employed after heating the sample so as to vaporize or to sublime. Thus the mixed gas forms the supersonic jet flow. In this case, the sample is adiabatically expanded, and consequently, the molecules are cooled. In this way, the rotational energy distribution of the sample molecules is remarkably simplified and the spectroscopic structures of the sample molecules are correspondingly simplified. In this point of view, such a method is suitable for quantitative as well as qualitative analysis.
There is another well-known method, which comprises steps as follows: spraying a sample in form of an ionic solution into atmosphere; impinging a large quantity of nitrogen gas on the sample mist to strip solvent molecules; introducing the solute ions into vacuum by applying an electric field so as to generate an ion beam, prior to measure the mass of the ions. According to this method, a capillary is combined with skimmer(s), and a differential pumping is applied to achieve high vacuum in a mass detection region located in downstream of the ion beam flow. In this way, a continuous ion molecular beam is obtained.
However, the first conventional method for generation of a molecular beam is ineffective for a liquid or solid sample with a high molecular weight. For examples, most of the protein molecules can be decomposed at a high temperature and most of polymer molecules cannot be sublimated or vaporized even at a high temperature.
The second conventional method to produce an ion beam is ineffective for neutral molecules, as it can be applied only for ionized molecules. The term “neutral molecules” used herein refers to non-ionic molecules.
In view of the problems as have been described above, it is a principal object of the present invention to provide a brand-new method and an apparatus to generate a neutral molecular beam. The apparatus as well as the method described in the present invention enable to generate a molecular beam for a wide variety of molecules, particularly for the molecules which can be decomposed by heating at a high temperature or those which can not be sublimated or vaporized even at a high temperature. In addition, with the method as well as the apparatus described in the present invention, it is possible to photo-ionize the neutral molecules and the inclusion cluster contained in the neutral molecular beam produced in this manner, for example, by irradiating with laser beams and thereby to carry out the mass spectroscopy studies and other spectroscopic analyses.